1. Field of the Invention
This invention relates to pneumatic radial tires for truck and bus mounted on 15.degree. drop center rims (hereinafter abbreviated as 15.degree. taper radial tire. More particularly it relates to a tubeless pneumatic radial tire for truck and bus adaptable to a drop center rim having a 15.degree. tapered bead seat (15.degree. drop center rim). Especially, the invention is concerned with a 15.degree. taper radial tire for truck and bus having advantageously improved durability of a bead portion by rationalizing an outer profile and a constituting material shape of the bead portion engaging with a flange of a rim.
2. Description of Related Art
As mentioned above, the 15.degree. taper radial tire for truck and bus is a tubeless (T/L) tire. In order to directly mount the tire of this type on a one-piece mold rim 21 having a section profile as shown in FIG. 20, there is applied a drop center rim (hereinafter referred to as 15.degree. drop center rim) in which a height of a flange 21F is made considerably lower than that of a width-wide flat base rim having a section profile shown in FIG. 21. Owing to the peculiar section profile of this flange, a bead portion of the T/L tire for truck and bus considerably differs from a bead portion of a tubed (W/T) tire to be mounted on the width-wide flat base rim. Further, the T/L tire is characterized by having a structure that a taper of about 15.degree. is applied to a bead base of the bead portion so as to match with a taper of 15.degree..+-.1.degree. formed on the bead seat of the rim 21 for ensuring air tightness and at the same time a given interference is applied to the bead portion.
As seen from the above description, although both the W/T tire and the T/L tire for truck and bus are used under the same heavy loading condition, a great difference in external force applied to the bead portion during the running under the heavy loading is naturally caused between both the tires. Because, the T/L tire having a lower height of the flange 21F becomes more unfavorable for fixation of the tire to the rim and the fall quantity of the bead portion toward the outside of the tire becomes larger as compared with that of the W/T tire. As the fall quantity increases, it is needless to say that problems with the bead portion are apt to be caused. Furthermore, with the W/T tire for truck and bus is difficult to attain the automation in the mounting onto rim, while the T/L tire is made possible to easily assemble on the rim by means of an automatic assembling device. As a result, the use of the T/L tire becomes more popular.
Therefore, various countermeasures as mentioned below have been taken up to the present time in order to improve the durability of bead portion in the T/L tire preventing the occurrence of bead portion trouble. These countermeasures are roughly divided into means for improving the rigidity of the bead portion and means for rationalizing the arrangement and shape of various cord layers extended from the bead portion to the sidewall portion.
As the former means for improving the rigidity, there are proposed (1) an increase of bead portion volume and additional arrangement of bead portion reinforcing cord layer, particularly means for the former volume increase in which since the bead core plays a basic role, a bead core having a low sectional height and a wide sectional width is arranged in correspondence with the low flange height of the aforementioned rim and a hard stiffener rubber extending taperedly outward in the radial direction of the tire is arranged from an outer peripheral surface of the bead core along a main body of a carcass ply to increase a volume of this rubber occupied in the bead portion as far as possible, (2) means for rendering an inner peripheral surface of a bead core having a flat and tetragonal section or a bead core having a flat and hexagonal section as mentioned in detail below into a tapered surface substantially matched with the surface of 15.degree. tapered bead seat to more strengthen the engagement of the bead portion with the rim, and the like.
As the latter means for rationalizing the arrangement and shape of the cord layer, there are proposed (3) means for adjusting heights of a turnup end position of a radial carcass ply and an end position of a bead portion reinforcing cord layer, or means for adjusting positions of these ends in the widthwise direction of the tire, (4) means for mitigating stress at an end portion of rubber surrounding the above ends, (5) means for rationalizing a radial carcass line (a line of thickness center of the main body of the carcass ply other than the turnup portion thereof), and the like.
All of the above means for improving the durability of bead portion have developed effects as they were. However, the above conventional means and their extensions become no longer impossible to cope with the present circumstances such as more long-lived tire performance, increase of demand for the formation of a recapped tire after the tread rubber is worn, severer demands for weight reduction and the like.
Particularly, the increase of the bead portion volume and the additional arrangement of the bead portion reinforcing cord layer are inappropriate for weight reduction. Furthermore, the excessive volume increase and additional arrangement bring about a greater amount of heat generation during the running of the tire under the heavy loading and hence the inside of the bead portion becomes a high temperature. Such a high temperature brings about the rubber deterioration and the adhesion degradation, whereby separation failure is apt to be caused from cracking at the turnup end portion of the carcass ply or the end portion of the bead portion reinforcing cord layer. Further, when the tire is used under condition of such a high temperature over a long period, the rationalization of the arrangement and shape of the cord layer with much effort is damaged by a large creep deformation of rubber as mentioned later and hence the given object can not be attained. Also, the shape of the carcass line is lost to bring about degradation of the steering stability.
Even when the increase of bead portion volume and the additional arrangement of bead portion reinforcing cord layer are within adequate ranges, the T/L radial tire for truck and bus mounted on the rim having a low flange height is naturally required to ensure a volume of the bead portion sufficiently durable to the heavy loading. When the tire having such a bead portion structure was run over a considerably longer distance such as 100,000 km, it has been confirmed that the bead portion creates a large creep (plastic) deformation as a whole. The state of this deformation is partially and sectionally shown in FIG. 22.
FIG. 22 is a section view of a main part of the bead portion when an assembly of T/L radial tire having a tire size of 11R22.5 and a rim is inflated under an internal pressure of 7 kgf/cm.sup.2 (under no load), in which a portion shown by a solid line is a new tire and a portion shown by a broken line is a tire after actual running over 100,000 km. As seen from FIG. 22, the bead portion after the running over 100,000 km creates a non-restoring creep deformation directed toward the outside of the tire.
In the tire after running (portion shown by the broken line), the hexagonal bead core causes a large shape lose and moves in a direction shown by arrows and hence the carcass ply and its turnup portion are largely pulled out in a direction shown by arrows therealong. Such a large pulling-out phenomenon always causes a large shearing strain represented by a lozenge at both sides of the turnup end portion in FIG. 22. As a result that the above large shearing strain under the inflation of internal pressure simultaneously acts together with shearing strain produced in the turnup end portion during the running under the heavy loading, a crack is first created at the turnup end portion and grows into the separation failure as the running distance becomes long. Therefore, even if it is intended to rationalize the aforementioned means for improving the durability of the bead portion, the sufficient bead portion durability can not be realized.